1. Field of the Invention
The present invention relates to a large scale integrated circuit device having a clock signal generating circuit built therein. In particular, the present invention is related to a large scale integrated circuit device which is capable of selecting either a CR oscillation circuit or a crystal oscillation circuit so that a user may use one of those oscillation circuits selectively according to the user's need.
2. Description of the Related Art
The inventors of the present invention know that there is a large scale integrated circuit device having a clock signal generating circuit built therein. This large scale integrated circuit device is arranged to select either a CR oscillation circuit or a crystal oscillation circuit by using metal layer masks respectively.
An arrangement of the above-mentioned known integrated circuit device is shown in FIG. 1. The integrated circuit device shown in FIG. 1 is composed of two kinds of metal layer masks, one kind of the metal layer masks being used for the CR oscillation circuit and the other kind of the metal layer masks being used for the crystal oscillation circuit, having respective circuit patterns as shown in FIGS. 2 and 3.
As shown in FIGS. 1 to 3, the above-mentioned known integrated circuit device includes a CR oscillation circuit 10, an oscillation inverter 11, a feedback resistor 12, a crystal oscillation circuit 13.
In the above-mentioned known integrated circuit device, the crystal oscillation circuit 13 includes the oscillation inverter 11 and the feedback resistor 12. Terminals 14 and 15 of the above-mentioned integrated circuit device are connected to a crystal oscillator (not shown) so that the integrated circuit device may output a predetermined signal.
In the connecting state shown in FIG. 2, the CR oscillation circuit 10 supplies an output signal as an internal clock CK. In the connecting state shown in FIG. 3, the crystal oscillation circuit 13 supplies an output signal as an internal clock CK as well.
However, above-mentioned known integrated circuit device requires two kinds of masks, one kind of masks for the CR oscillation circuit and the other kind of the masks for the crystal oscillation circuit in order to select either the CR oscillation circuit 10 or the crystal oscillation circuit 13. Moreover, a user must have two kinds of integrated circuits in order to use both of the CR oscillation and the crystal oscillation since they use different kinds of masks.